In addition to the answer provided by amelingu I offer the following comments.
1. From what information you have supplied the primary computer should run the GTX980ti card as it has 6Gb of GPU. The more GPU the better with X-Plane. In addition make sure the 32Gb Ram is installed in the primary computer. This is more than enough. X-Plane is GPU and CPU hungry. This combination will give you better results than a mix and match with the lesser RAM and lower spec (GPU) graphics card.
Connect the 3 screens to this graphics card. If your 3 screens are the same brand and exactly the same model you will get the best effect. If they are G-Sync compatible you will get better results again.
Any instruments you have should be connected to the primary computer.
2. Now for the secondary PC. You don't need to run a second copy of X-Plane Use the GTX 780 card and the 16GB of RAM in conjunction with the touch screen using a product called Air Manager. This second computer is an overkill for the product.
3. Have a look at several videos produced by Michael Brown which demonstrates the use of the product with the touch screen and multiple monitors. The link can be be found at https://www.youtube.com/user/col2mab/videos?sort=dd&view=0&shelf_id=0 Laminar Research recommends Michael as the X-Plane PC supplier in the USA. Both businesses are in the same town. I give you the full link for his videos because he has made several with Air Manager and uses the Touch screen for his instruments. You will need to network the two computers. Michael makes comment about this in one of his videos.
4. Air Manager can be found at the following link https://siminnovations.com/ An alternative is Panel Builder. I have this product and will most like change to Air Manager. Air Manager seems to be the recommended product to use. Some time back Michael Brown was a Panel Builder user and has now made the switch.
5. If having trouble in networking your 2 PCs have a look at the following 2 videos. The producer is a long time FSX user but has a lot of good and interesting information. The two links can be found at https://www.youtube.com/watch?v=oMx3pl3Z3Mg and https://www.youtube.com/watch?v=c3FyjMDCJ0U
Finally, I do not have any connection with any of the products and or companies mentioned above. Rather difficult down under.
Hope this all helps Enjoy the setup and good flying.